Assignment 12

High Voltage Section

Introduction

The TV’s high voltage section provides the very high voltage needed by the CRT. This report will discuss the common problems and possible solutions in this section of the Television receiver.

Symptoms and Troubleshooting

The high voltage section symptoms vary, but the most common ones are loss of raster and high voltage, but sound is working, blooming, corona, overloading the horizontal output amplifier and arcing.
When the raster and the high voltage is lost, it means a loss in either DC or AC supply to the HV section. One can only use a HV probe or a neon indicator to check for voltages in this section. A defective high voltage rectifier or an open filter resistor can cause this,
Blooming is the condition in which an increase in brightness causes a simultaneous raster expansion and a loss of focus. This is caused by poor high voltage regulation. This translates that the high voltage decreases as the picture tube beam current increases. This may be causes by a bad diode, a defective picture tube, damper, flyback transformer or defective hold down circuits.
A defective horizontal output transistor causes loss of raster. This defect may have generated from a shorted flyback transformer.
Corona is the result of ionization of the air, caused by very strong electrostatic fields generated by high voltage circuits. It’s known by a hissing sound and a bluish glow surrounding the point of emission. The high voltage should be measured and corrected to fix this problem. Arcing is an electrical breakdown when two points are linked by a jumping spark, causing a loud snapping sound and a flash of light. Its eliminated by putting HV putty on the two places.
To troubleshoot the Flyback Transformer, one should ground the chassis, unplug the power cable and smell the transformer. When it’s burnt, it smells like burnt paraffin. Remove the transformer from the circuit and use an ohmmeter to check each pin for the expected values. This procedure will determine whether its faulty.
With shorted components, which result with no raster, burning out is easily visible, For dim, out of focus or horizontally narrow pictures, check for leaky devices which cause a reduction in voltage. Check the output of the horizontal output circuit, if the voltage is low, there must be a leaky transistor. If a transistor is open, there will be no high voltage, the fuse will blow and the television will shut down. If there is no input at the base of the of the HV transistor, check the horizontal circuits.
The bleeder resistor is connected next to the flyback transformer (usually) and its used to stabilize the high voltage in the television, keeping the focus voltage constant. If the focus voltage is incorrect, the picture will not be correctly focused. If the focus voltage is not present at all, then there’s no raster. This means that one has to replace both the resistor and the flyback transformer. The CRT should be discharged before working on the flyback transformer.

Reference:

• Howard W. Sams, Troubleshooting and Repair Guide to TV, 1st Edition, ©1996 by Howard W. Sams and Company, Prompt Publications, Indianapolis, Indiana. Pg 150-158
• Liff, A. Alvin, Wilson, Sam J.A. Color and Black and White Television, Theory and Servicing, 3rd Edition © 1993 by Prentice Hall, Inc, A Pearson Education Company, Upper Saddle River, New Jersey, pg 445-451